This invention is directed to a liquid atomizing and spraying device which is used, for example, in ordinary spray painting and electrostatic coating.
Among the liquid atomizing and spraying devices (hereinafter referred to as spraying devices) which have been known and used in electrostatic coating apparatus, the following two types are the most widely used.
(1) The first type has a means for supplying coating fluid such as varnish or paint (hereinafter referred to representatively as paint) in the form of a thin film into, for example, a cup-like rotating member which is being rotated at a constant speed and a means for impressing a high direct-current voltage between the front extremity of the cup-like rotating member and the work being coated thereby atomizing the paint with electrostatic field and causing the atomized paint particles to be deposited on the work.
(2) The second type has an orifice having an annular shape, a means for supplying paint to this orifice, a means for supplying compressed air around the outer periphery of the orifice thereby atomizing the paint with an air jet, and a means for impressing a high direct-current voltage between the work being coated and the front extremity of the orifice thereby causing the paint to be deposited on the work with excellent deposition efficiency.
However, in the electrically atomizing means of the circular rotating member of the first type, the paint spreads in the form of a thin film over the surface of the circular rotating member and, thus being exposed to air, dries and adheres thereto. For this reason, it is necessary to wash the surface of the circular rotating member over which the paint film is flowing every time the coating operation is stopped, and it becomes difficult to change colors in a short time. Furthermore, since the atomization is accomplished by electric force, sufficient atomization cannot be obtained in the case of paints having a low electrical resistance such as water-soluble paints and a coating effect due to electrostatic field cannot be practically achieved.
Furthermore, in the spraying device of the previously described second type which depends on air-jet atomization by an orifice, paint particles accumulate at the front head part of the sprayer because of the state of the air flow and give rise not only to poor coating of the work, but also to clogging of the sprayer orifice.
In general, atomization of a paint having a low electrical resistance, such as water-soluble paints, by means of an air-jet atomizing device is made possible by increasing the pressure of the compressed air or by heating the paint and lowering the surface tension thereof. However, increasing the velocity of the air stream in an electrostatic coating apparatus leads to a lowering of the deposition efficiency, the result being that it is difficult to manufacture an electrostatic coating apparatus of high efficiency.
Many research reports have been issued in recent years on processes and mechanism for atomization or fine subdivision of bodies of liquids. According to these disclosures, in the case of electrical atomization with a circular rotating member, a liquid film is formed up to the edge of the circular member and extends from the edge to generate flares, which are subjected to vibration produced by an electrically generated force and split into a mesh form. It has been found that, in the case of atomization of a liquid film due to the air flow in the above described orifice means, as the compressed air pressure is increased, the liquid film which is ejected out through the orifice, forms cusps at the forward extremity of the orifice and clearly splits into a mesh-form. It is further known that in the case of a circular rotating member which is supplied with paint at a constant flow rate, when the impressed voltage is determined, the number of ligaments generated increases with a corresponding increase in the rotational velocity of the rotating member. In the case of atomization by an air jet, it has also been experimentally found that the number of cusps generated increases as the compressed air pressure increases.
However, there is a limit to the increase of the rotational velocity of the rotating member, and the increase of the compressed air pressure is also subject to limitations due to the mechanism employed. For these reasons, the number of cusps generated is limited in all cases by the surface tension of the paint at the liquid film discharge extremity.